Bonded magnets have been conventionally used various extensive applications such as electrical appliances and automobile parts owing to a good shape adjustability and a high dimensional accuracy. In recent years, with the tendency toward reduction in size and weight of these electrical appliances and automobile parts, it has been strongly required that the bonded magnets per se used therein have a high performance and a high corrosion resistance capable of withstanding severe environmental conditions.
The bonded magnets have been in general produced by kneading magnetic particles together with a binder resin such as rubbers and plastic materials and then molding the resulting kneaded material. Therefore, in order to obtain the bonded magnets having a high performance, it has been strongly required that the magnetic particles used therein have a high performance, i.e., exhibit a large residual magnetic flux density Br and a high coercive force iHc and as a result, a large magnetic energy product (BH)max.
As the magnetic particles, there are known magnetoplumbite-based ferrites such as barium ferrite and strontium ferrite, Nd—Fe—B-based magnetic particles and Sm—Fe—N-based magnetic particles.
The Nd—Fe—B-based magnetic particles have been extensively applied to high-efficiency motors owing to both a high saturation magnetization and a high anisotropic magnetic field thereof. Sintered magnets have been extensively used in the applications including not only mobile phones and various domestic appliances but also large-scale magnetic circuits for magnetic medical diagnosis equipments (MRI), radiation generators and the like. The bonded magnets have been used in the applications including spindle motors for CD, DVD and HDD, vibration motors for mobile phones, actuators for digital cameras, etc. In addition, studies have been made to apply these magnets to automobile parts for the purposes of weight reduction, energy saving and improved performance thereof.
The Sm—Fe—N-based magnetic particles have both a high saturation magnetization and a high anisotropic magnetic field similarly to the Nd—Fe—B-based magnetic particles. In addition, the Sm—Fe—N-based magnetic particles also have a high Curie temperature and therefore have been recently noticed. In particular, the Sm—Fe—N-based magnetic particles have a higher rust prevention property than that of the Nd—Fe—B-based magnetic particles. Therefore, it has been expected that the Sm—Fe—N-based magnetic particles are used under severe environmental conditions in which bonded magnets formed of the Nd—Fe—B-based magnetic particles are not usable.
The Nd—Fe—B-based magnetic particles may be produced, for example, by the method in which an alloy mass comprising neodymium, iron and boron is treated at an elevated temperature in a hydrogen atmosphere to once decompose the alloy into a rare earth hydride, and an Fe compound and an Fe—B compound, i.e., subject the alloy to hydrogenation and disproportionation (HD treatment), and then remove hydrogen from the resulting particles to obtain purified fine compound crystals again (DR treatment). However, it is required that the size of the thus obtained Nd—Fe—B-based magnetic particles is adequately adjusted in order to apply the magnetic particles to a magnet. Therefore, the Nd—Fe—B-based magnetic particles must be subjected to crushing treatment at least to a minimum extent. The crushing treatment tends to however cause exposure of an active surface of the respective magnetic particles to outside, so that the Nd—Fe—B-based magnetic particles tend to suffer from promoted oxidation owing to exposure of the active surface. In particular, the Nd—Fe—B-based magnetic particles tend to be readily oxidized for a short period of time in a wet air, resulting in deterioration in magnetic properties thereof. Further, when subjected to a kneading step with a resin and a molding step, the Nd—Fe—B-based magnetic particles tend to suffer from deterioration in magnetic properties thereof owing to an oxidizing or reducing atmosphere used in these steps or heat generated therein. In addition, the Nd—Fe—B-based magnetic particles are very likely to be rusted owing to inclusion of Fe. In the case where a bonded magnet formed of the Nd—Fe—B-based magnetic particles is used in corrosive environmental conditions such as sea coast, the bonded magnet tends to suffer from formation of rusts even when the bonded magnet is produced using a low water-absorbing resin.
On the other hand, the Sm—Fe—N-based magnetic particles may be produced by occlusion of nitrogen into an alloy of samarium and iron. The size of Sm—Fe—N-based magnetic particles must be adequately adjusted in order to obtain a permanent magnet therefrom. Therefore, the Sm—Fe—N-based magnetic particles must also be subjected to crushing treatment at least to a minimum extent. The crushing treatment tends to however cause exposure of an active surface of the respective magnetic particles to outside, so that the Sm—Fe—N-based magnetic particles tend to suffer from promoted oxidation owing to exposure of the active surface. In particular, the Sm—Fe—N-based magnetic particles tend to be readily oxidized for a short period of time in a wet air, resulting in deterioration in magnetic properties thereof. Further, when subjected to a kneading step with a resin and a molding step, the Sm—Fe—N-based magnetic particles tend to suffer from deterioration in magnetic properties thereof owing to an oxidizing or reducing atmosphere used in these steps or heat generated therein. Also, although the Sm—Fe—N-based magnetic particles are less rusted as compared to the Nd—Fe—B-based magnetic particles, the Sm—Fe—N-based magnetic particles tend to be decomposed at an elevated temperature. Therefore, the Sm—Fe—N-based magnetic particles tend to be used together with only a low-melting resin such as epoxy resins and polyamide resins when forming a bonded magnet therefrom, and therefore tend to gradually absorb water to generate rusts therein. For example, when used in corrosive environmental conditions such as sea coast, the Sm—Fe—N-based magnetic particles tend to be suffer from formation of rusts. When kneaded with super-engineering plastics having a high melting point, the Sm—Fe—N-based magnetic particles tend to be considerably deteriorated in coercive force, thereby failing to obtain a bonded magnet having magnetic properties as aimed.
Thus, it has been strongly required to provide the Nd—Fe—B-based magnetic particles and the Sm—Fe—N-based magnetic particles which are less deteriorated in magnetic properties thereof even when exposed to an oxidizing or reducing atmosphere or heat generation which will be caused in each of drying, surface-treating, kneading and molding steps, and a bonded magnet formed of these magnetic particles which are hardly rusted even when used in corrosive environmental conditions.
Also, a moldability of the bonded magnet is an important property upon practical use, but tends to vary depending upon a fluidity of a mixture of the magnetic particles and a resin under high-temperature and high-pressure conditions. Therefore, it is important that the magnetic particles have a good resistance to chemical reactions upon molding with the resins.
There is conventionally known a method of surface-treating Nd—Fe—B-based magnetic particles to enhance an oxidation resistance thereof in which the magnetic particles are coated with a phosphoric acid-based compound (Patent Document 1). Also, there is known a method of forming an SiO2 protective film on the respective Nd—Fe—B-based magnetic particles (Patent Document 2).
There is also known a method of surface-treating Sm—Fe—N-based magnetic particles to enhance an oxidation resistance thereof in which the magnetic particles are coated with a phosphoric acid-based compound (Patent Document 3). In addition, there is known a method of surface-treating Sm—Fe—N-based magnetic particles to enhance an oxidation resistance thereof in which a silica coating film is formed on the respective magnetic particles (Patent Documents 4 to 6). Further, there is known a method in which after Sm—Fe—N-based magnetic particles are coated with a phosphoric acid-based compound, a silica coating film is further formed on the respective coated magnetic particles (Patent Documents 7 and 8).
Patent Document 1: Japanese Patent Application Laid-Open (KOKAI) No. 2006-49863
Patent Document 2: Japanese Patent Application Laid-Open (KOKAI) No. 8-111306
Patent Document 3: Japanese Patent Application Laid-Open (KOKAI) No. 2000-260616
Patent Document 4: Japanese Patent Application Laid-Open (KOKAI) No. 2000-160205
Patent Document 5: Japanese Patent Application Laid-Open (KOKAI) No. 2000-309802
Patent Document 6: Japanese Patent Application Laid-Open (KOKAI) No. 2005-286315
Patent Document 7: Japanese Patent Application Laid-Open (KOKAI) No. 2002-8911
Patent Document 8: Japanese Patent Application Laid-Open (KOKAI) No. 2002-43109